1. Field of Invention
The present invention relates to scanning probe microscopy systems, in particular, atomic force microscopy systems comprising carbon nanotube tips, and methods for fabricating such tips.
2. Discussion of the Related Art
Scanning probe microscopy (SPM) such as atomic force microscopy (AFM) has been an important and powerful technique for resolving nanoscale features, and thus has been utilized for various scientific, engineering, and biological applications. The key component of SPM is the probe tip, as the resolution of SPM imaging is determined by its sharpness, size and shape. See articles by G. Reiss, et al, “Scanning tunneling microscopy on rough surfaces: tip-shape-limited resolution”, J. Appl. Phys. 67, 1156 (1990), and by J. E. Griffith et al, “Scanning probe metrology”, J. Vac. Sci. Technol. A 10, 674 (1992).
Typical commercially available SPM probe tips are made of silicon or silicon nitride microfabricated into a pyramid configuration. Such probes are often easily broken or worn out during long time operation. They also generally exhibit a limited lateral resolution, and their rigid pyramid shape does not allow easy access to narrow or deep structural features.
Carbon nanotubes (CNTs) have attracted much attention due to their various interesting physical and chemical properties. The high aspect ratio geometry and the nano-scale diameter of the CNT offer obvious advantages for imaging as an AFM probe. Moreover, due to its good mechanical flexibility, such a CNT probe is also suitable for studying soft matters such as biological samples with minimal damage.
Carbon nanotubes (CNTs), either single wall carbon nanotubes (SWNTs) or multiwall nanotubes (MWNTs) can be grown in a controlled manner using chemical vapor deposition (CVD) processing. Carbon nanotubes with graphene walls parallel to the axis of the nanotube as well as those with graphene walls at an angle to the axis of the nanotube can be grown. The latter type of carbon nanotubes, sometimes called nanofibers, still have a nanoscale tube configuration, and hence will be referred throughout this disclosure as nanotubes. Vertically aligned, periodically spaced MWNTs can be grown in a controlled manner using DC-plasma enhanced CVD process using an applied electric field. See V. I. Merkulov, et al, Appl. Phys. Lett. 80, 4816 (2002), J. F. AuBuchon, et al, Nano Letters 4, 1781 (2004).
There have been several approaches developed for fabrication of CNT based probes. Most approaches are based on attaching CNTs (mostly multiwall nanotubes) on commercial pyramid tips by acrytic adhesive, electric field, arc welding, magnetic field and liquid phase dielectrophoresis. See articles by H. Dai, et al, Nature 384, 147 (1996), H. Nishijima, et al, Appl. Phys. Lett. 74, 4061 (1999), by R. Stevens, et al, Appl. Phys. Lett. 77, 3453 (2000), by A. Hall, et al, Appl. Phys. Lett. 82, 2506 (2003), and by J. Tang, et al, Nano Lett. 5, 11 (2005).
These methods are operated manually and are time consuming. The attachment angle, the number of CNTs attached, and adhesion strength are not always controllable. A direct growth of CNTs with catalyst particles or catalyst film coating on Si tips by thermal CVD has also been reported. See articles by J. H. Hafner, et al, Nature 398, 761 (1999), by C. L. Cheung et al, Appl. Phys. Left. 76, 3136 (2000), and by E. Yenilmez, et al, Appl. Phys. Lett. 80, 2225 (2002). While such an approach can potentially lead to wafer scale production of AFM tips, there are some major issues that need to be resolved for practical SPM applications to materialize:
i) The reproducibility and reliability in shape, size, and attachment angle of nanotube probes is yet to be established. Snow et al. reported the effect of the attachment angle of the CNT tip with respect to the cantilever body for AFM imaging. It was shown that a tilt attachment angle of the CNT can severely reduce the imaging performance. See E. S. Snow, et al, Appl. Phys. Lett. 80, 2002 (2002).
ii) The frequent presence of undesirable multiple nanotubes at the probe tip, instead of a desirable single nanotube is a problem. This is often seen during the prior art in-situ CVD growth of nanotubes from AFM pyramid tips, due to the presence of multiple catalyst particles, as it is not always easy to place just a single catalyst island at the pyramid apex. The presence of such multiple nanotubes at the probe tip, some of which tangle with each other, is highly undesirable as it complicates the AFM imaging and interpretations.
iii) To ensure a suitable length of CVD-grown CNTs for imaging, an electric pulse cutting technique has been developed to shorten CNT on the tip during AFM operation. This process is tedious because every CVD-grown CNT tip has to be checked and trimmed individually. See J. H. Hafner, et al, Nature 398, 761 (1999).
iv) The attachment or growth of too small a diameter nanotube such as a single wall nanotube (SWNT) with a diameter of ˜1.2 nm induces an instability problem, especially if the probe length is made reasonably long for ease of probe handling and fabrication as well as for ease of access into deep cavities. Such a thin and long probe tends to vibrate with high frequency, thus the lateral position and resolution of the probe tend to get deteriorated.
Therefore there is a need to find an improved SPM or AFM probe configuration and fabrication technique in order to resolve these serious issues. This invention discloses a simple, reliable and protection-layer-free technique of fabricating a single SPM probe on the cantilever and unique probe tip structures by utilizing the unique direct-write feature of the electron-beam-induced carbon island deposition. Electron beam induced deposition (EBID) of carbon is a novel writing technique to directly fabricate nanopatterns on the substrate bypassing the use of any resist-layer-related steps. The technique is especially useful for creating a pattern on sample substrate edges, especially on tiny samples such as a pre-fabricated tipless cantilever. The resultant formation of a nano island metal catalyst pattern allows a growth of a sharp, high-aspect-ratio nanotube probe structure with desired mechanical stability by an electric-field-guided CVD process.